
No federal bureaucrat played a bigger role in 20th-century law enforcement than J. Edgar Hoover (1895-1972), who served as the head of the FBI and its predecessor agency for half a century.
Hoover oversaw crackdowns on everything from real and imagined communists in the first Red Scare of the 1920s and its sequel in the 1950s; staged high-profile shootouts with "public enemies" like John Dillinger and Babyface Nelson in the 1930s; surveilled Nazi and Axis sympathizers during World War II; infiltrated the Ku Klux Klan in the 1960s; and pursued extra-legal operations against civil rights leaders and antiwar protesters in the 1960s.